Intraspecific variation in muscle and organ growth in three strains of chicken with differential genetic selection for fast growth rate

Abstract

Intensive genetic selection for fast growth rate in chickens over the last 40 years has halved the time needed for broilers to reach 2 kg in weight (Gyles, 1989), with most of this growth difference occurring within the first 2 weeks of life (Ricklefs, 1985). We investigated the effect of this extensive selection on organ morphology in 3 strains of chicken that have undergone differential rates of selection. The strains include: a modern commercial broiler (Ross 308, FB), a broiler that has not been selected for fast growth since 1972 (Ross 1972, SB) and a layer chicken which has not been selected for fast growth rate (Euribrid HISEX, L).Each strain was grown to the point of maximum growth rate which was at 42, 56 and 112 d for the FB (n= 52), SB (n= 64) and L (n= 120) strains respectively. One-d-old chicks were sexed, wing-banded and reared as separated sexes in pens on a wood-shavings litter. Each strain was grown to standard commercial practice for environmental temperature and lighting regimen. The photoperiods were 23L:1D for broiler strains and 8L:16D for the layer strain. Both strains of broiler were fed standard commercial broiler rations. The layer chicks were divided into 2 dietary groups, 1 group was fed the same rations as the broilers and the 2nd group was fed commercial replacement pullet rations. Water and food were available ad libitum. Individual body weights (BM) were measured daily for the 1st week and weekly thereafter. A random sub-sample of chicks was removed at the same time intervals for serial morphological measurements. Birds were killed by cervical dislocation and weighed. The wet masses of the gizzard, intestine, caeca, heart, liver, brain, lungs, pectoral muscle and leg muscle (surrounding the femur and tibiotarsus) were measured. All tissues were dried to constant mass in an oven at 60°C. Statistical analyses were performed using ANCOVA (Minitab 10·5) and linear regression (Genstat 5), where non-linear data were transformed. There was a very high degree of correlation between tissue masses and body mass, so the mass of the tissue under consideration was subtracted from BM.As expected, both growth rate and BM were significantly different between all 3 strains of chicken and between sexes in each strain. There was no significant difference in BM or organ masses between the 2 dietary groups of L chickens, so these data were pooled for subsequent analysis. All 3 strains of chicken were grown until growth rate peaked which was approximately; 700 g/week by d 35 for FB, 275 g/week by d 49 for SB and was around 175 g/ week by d 63 for the L strain. There was a significant effect of strain (P£ 0·01) on the masses of the following tissues; pectoral muscle, leg muscle, gizzard, intestine, liver, heart, lungs and brain, when the effect of dry body mass was taken into account. However, there was no significant strain effect on the mass of the caeca. There was an interaction between strain and age for all tissues (P£ 0·001). The masses of both pectoral and leg muscle increased with bird age in a ...

Searching for causal relationships among latent variables concerning performance, carcass, and meat quality traits in broilers

“…Different studies have related the high growth rate in broilers with an excess of abdominal fat in addition to physiology, performance, and development changes of internal organs (Gaya et al, 2006;Rance et al, 2002;Venturini et al, 2014). When evaluating the variation in muscle and organ growths in different strains of chickens, Gavin and McDevitt (1999) reported that chickens selected for faster growth (fast-growing broilers) showed heavier muscle and organs masses compared to those selected for slower growth (slow-growing broilers). However, the proportion of organ mass to muscle mass was lower in fast-growing broilers.…”